1. Field of the Invention
This invention relates to rapidly solidified ultrafine iron-neodymium-boron alloys obtained by adding small amounts of transition metal borides. This invention also relates to the preparation of these materials in the form of rapidly solidified filaments and particulates which are suitably heat treated and then communited into fine powders. The fine powders are aligned inside an applied magnetic field followed by pressing and sintering into near fully dense magnets possessing high energy products.
2. Description of the Prior Art
Permanent magnet materials are distinguished by microstructures including two magnetically different phases on an extremely fine scale, as in the Alnicos and Fe--Cr--Co alloys, high magnetocrystalline anisotropy, as in Co--Sm and the barium ferrites, or both, as in the Cu-modified cobalt-rare-earths and their descendants. These microstructures result from various processing and heat treatment procedures. Such structures can also be produced by crystallizing amorphous alloys or directly by rapid quenching. These processes lead to fine-scale heterogeneity and can also result in the production of phases, for example, Fe.sub.3 B, (see J. J. Rhyne, J. H. Schelleng and N. D. Koon in Physical Review. B10, pp. 4672, 1974) that would not be stable under more nearly equilibrium conditions. Such phases may have low symmetry and possibly high magnetocrystalline anisotropy. For all these reasons crystallized amorphous materials seemed attractive to explore for potential permanent magnet properties.
The high coercive forces and energy products (BH).sub.max among commercial permanent magnet materials are found in cobalt-samarium alloys. The high coercivity results from the very high magnetocrystalline anisotropy that can occur in intermetallic compounds containing transition metals and rare earths. In search of new cobalt and samarium-free permanent magnet materials, the early studies (see R. C. Taylor in J. Appl. Physics, 47, pp. 1164, 1976) have been made on amorphous RFe.sub.2 (R=rare earth) prepared by rapid quenching having large coercivities at cryogenic temperatures. Since then similar behavior has been observed in other rare earth systems (see A. E. Clark in Appl. Physics Letter, 23, pp. 642, 1973 and J. J. Croat in Appl. Physics Letter, 37, pp. 1096, 1980). The philosophy of the technical approach is to utilize the wide range of metastable microstructures accessible by rapid quenching at controlled rates followed (if desired) by heat treatment. The hard magnetic properties of these amorphous materials have been observed to increase with crystallization and Clark obtained a coercive field of 3.4 kOe and an energy product of 9 MGOe in TbFe.sub.2 at room temperature. More recently Koon et al (see N. C. Koon and B. N. Das, Appl. Physics Letter, 39, pp. 840, 1981) have observed high coercive fields in (Fe.sub.80 B.sub.20).sub.90 La.sub.5 Tb.sub.5 alloys crystallized from amorphous state. Continuing this effort Croat (see J. J. Croat, in J. Appl. Physics, 53, pp. 3161, 1982) produced high coercive fields in rapidly solidified ribbons of R.sub.40 FE.sub.60 alloys. Hadjipanayis et al (see G. C. Hadjipanyis, R. C. Hazelton, and K. R. Lawless, J. Appl. Phys. 55, pp 2073, 1984) investigated magnetic properties of rapidly quenched ribbons of FeRM alloys where R=La,Y,Pr,Nd,Gd and M-B,Si,Al,Ga,Ge over a wide range of chemical compositions. The alloys are generally magnetically soft in as-quenched amorphous state. Magnetic hardening is produced by crystallizing the amorphous phase by heat treatment at 700.degree. C. The best properties have been obtained in alloys based on iron-neodymium-boron (Fe--Nd--B) and iron-praseodymium-boron (Fe--Pr--B) systems. The hard magnetic properties of these materials are attributed to a highly anisotropic phase. X-ray diffraction and transmission electron microscopy (TEM) indicate that the high energy product alloys in the R--Fe--B systems crystallized from amorphous state consist of an extremely fine grained equilibrium phase. This phase is R.sub.2 Fe.sub.14 B according to Croat et al (see J. J. Croat, J. F. Herbst, R. W. Lee and F. E. Pinkerton, in 29th Annual Conf. on Magnetism and Magnetic Materials, Pittsburgh, PA, November, 1983). Other researchers have identified the stoichiometry of this phase to be R.sub.3 Fe.sub.16 B, R.sub.3 Fe.sub.20 B, or R.sub.3 Fe.sub.21 B (see G. C. Hadjipanyis, R. C. Hazelton and K. Lawless, J. Appl. Phys. Lett. 43, pp. 797, 1983). The hard magnetic phase has a tetragonal crystal structure with lattice constants a=8.8A and c=12.2A. The Curie temperature of this phase is 600 K..degree.. The transmission electron microscopy results showed that the particles composing the magnetically hard samples in R--Fe--B alloys are roughly spherical with diameter ranging from 20 to 100 nm. Croat et al (see J. J. Croat et al in 29th Annual Conference on Magnetism and Magnetic Materials, Pittsburgh, PA, November 1983) estimated a range of 80-100 nm for the single domain particle diameter using the observed Curie temperature and estimates of the exchange and anisotropy energies. The high coercivity mechanism is attributed to the effects due to the single domain particle. Limited studies of the effect of heat treatment variables have shown that the magnetic hardness to be a sensitive property of the anneal temperature. With increasing heat treatment temperature, the particle size of the hard magnetic phase in the crystallized alloy increases, leading to decrease in the coercivity due to multidomain effects.
Although promising permanent magnet alloy compositions have been identified in the light rare earth-iron-boron systems which can be prepared into bulk magnets from powders by various powder metallurgy processing techniques producing energy product values at room temperature ranging between 10 to 35 MGOe (see M. Sagawa, S. Fijimura, N. Togawa, H. Yamamoto and Y. Matsuura, J. Appl. Phys. 55, pp. 2083, 1984), these magnets suffer from certain drawbacks. They possess low Curie temperature (T.sub.c), and exhibit rapid decrease in coercivity (H.sub.c) and magnetization moment with increasing temperature. The poor thermal stability of iron-rare earth-boron magnets have prevented their widespread commercial use. In order to increase the application temperatures or in other words, the thermal stability of the Fe--Nd--B magnets, H.sub.c and/or T.sub.c of the alloys must be enhanced by one way or the other. Substitution of iron by cobalt results in increasing Curie temperature with a decreased coercivity (see M. Sagawa, S. Fujimura, H. Yamamoto, Y. Matsuura and K. Hiraga in IEEE Trans. Magn. MAG.20 pp. 1584, 1984). Heavy rare earths such as dysprosium or terbium increases coercivity but reduces remanent magnetization (see Y. Yang, W. James, X. Li and H. Chen in IEEE Trans. Magn. MAG.22, pp. 757, 1986). Partial substitution of iron with aluminum has been found to increase coercivity while reducing remanent induction and Curie temperature (see Z. Maocai, M. Deging, J. Xiuling and L. Shigiang in Proc. 8th International Workshop on REPM, Dayton, OH, pp. 554, 1985). Most efforts in the prior art so far have failed to enhance the thermal stability of Fe--Nd--B band permanent magnets. Therefore, there is a need to develop new processing methods to fabricate Fe--Nd--B alloys modified with suitable alloying elements which lead to desirable structures and magnetic properties of such alloys with enhanced thermal stability.